This invention relates to composites of a synthetic resinous material having randomly dispersed therein conductive fibers and a particulate conductive or semi-conductive filler material. Such composites are effective, for example, as shields for electro-magnetic interference (EMI) and/or electrostatic dissipation.
Electronic devices, particularly solid state electronic devices such as are found in computers, microprocessors, calculators, watches, radios, televisions, automobile ignition systems, word processors, and the like, are often sensitive to EMI which is present in the environment from many sources. It is most commonly emitted by electrical sources or the electronic devices themselves. Radio, television and other communication systems are also sources of EMI. EMI often disrupts the functioning of said electronic devices, causing diminished performance or even total failure of the device. Although normal performance of the electronic device is usually restored upon elimination of the EMI, the temporary failure of the electronic device may be of critical importance. For example, automobile electronic ignition systems have been known to fail due to EMI emitted from the spark plugs of the automobile's ignition system or even from the operation of the car radio. Such failure causes the entire automobile engine to be temporarily inoperative. Similarly, aircraft electronic guidance systems, communications systems, onboard computers and the like are known to be adversely effected even by the playing of portable radios in the aircraft.
To reduce these problems, it is common practice to "shield" electronic devices from external EMI. Metals are extremely effective shielding materials for EMI. Thus, it is known to protect electronic devices by placing a metal shield between the device and the external EMI source. This metal shield ranges in thickness from a foil to a load-bearing metal sheet.
It is often desirable from a design and economic standpoint to combine the EMI shielding function with structural or other functions. For example, if the EMI shield can be incorporated into other necessary parts of the article containing the electronic device, it is often possible to reduce the overall cost of the article by eliminating a part. While metal shields can often be used to combine functions in this manner, often the use of metal parts has decided disadvantages. Where weight is a factor, metal parts are often too heavy. Furthermore, metals cannot be molded into highly convoluted shapes. When a lightweight or highly complex shaped part is desired, it is therefore preferable to use a plastic material.
It has previously been attempted to provide resinous materials having EMI shielding properties. For example, it has been attempted to coat by painting, vapor deposition, electroless deposition and other techniques, a thin metallic layer onto a synthetic resinous material substrate. While good EMI shielding can be obtained in this manner, the coated surface is often subject to scratches, imperfections, marring, denting, etc., which opens "windows" to EMI. Environmental weathering and surface oxidation also adversely affect the metal layer. In addition, the coated surface often cannot be molded or shaped without destroying the integrity of the coating. Therefore, the synthetic resinous material must usually be pre-shaped in one step and subsequently coated in a separate step.
It has also been attempted to place a conductive material inside a synthetic resinous material part to form an EMI shield. Such conductive composites generally comprise a thermosetting or thermoplastic matrix containing carbon black or metal powders, flakes or fibers as a conductive filler material. While adequate shielding is often obtained with such composites, high loading of the conductive filler material is required to achieve the desired shielding. Moreover, these composites exhibit poor surface characteristics and are not highly formable (i.e., can be formed only at low extension ratios). In addition such composites generally are of high density and exhibit relatively low strength-to-weight ratios. Due to the high content of the filler material in such composites, molding equipment used to process these composites exhibit excessive wear due to the highly abrasive nature of the conductive filler materials.
Recently, metal fibers, metallized glass fibers, graphite and metallized graphite fibers have been proposed for use in composite EMI shielding materials. However, in the bulk molding or injection molding applications in which such composites are employed, breaking of the fibers greatly reduces the efficiency of the shielding. Accordingly, continuous metal or metallized glass fibers are employed to minimize the effect of the breaking, or it is necessary to conduct the molding operation under careful conditions in order to minimize such breaking. In either case, such composites did not provide an inexpensive means for providing an effective EMI shielding material. Moreover, due to the use of long fibers and thermosetting resins, such composites were not readily moldable as are thermoplastic resins. Due to the breakage of fibers in this composite, a content of metal fibers or metallized glass fibers of at least 25 percent by weight of the composite was generally required to provide effective EMI shielding. Since metal fibers and metallized glass fibers do not provide substantial reinforcement to the composite, it is generally necessary to add additional reinforcing fibers to obtain the desired physical properties. The resulting composite is a very dense material having poor moldability.
It is also often possible to provide a resin sheet which is electrically conductive. Such electrically conductive sheets would be capable of dissipating static electricity, thereby making them useful in preventing instantaneous discharge of built-up static electricity. Also, by dissipating static electricity, it is possible to reduce or eliminate electrostatic dust build-up on the sheet. Unfortunately, the presently available conductive resin sheets have the same types of deficiencies described for the EMI shield materials.
It would now be desirable to provide a synthetic resinous material composite sheet which is capable of converting radiation energy to heat and in which the disadvantages of previously known microwave absorbers are minimized or overcome.
It would also be desirable to provide a synthetic resinous material composite sheet which is useful as an EMI shielding material and which is also useful as a sheet that dissipates static electrical charges and/or absorbs microwave energy. Such a sheet should be readily moldable and possess good physical properties.